Heat contact welding is traditionally used for embossing, welding, cutting, and producing of bending lines in thermoplastic material, said welding involving a direct or an indirect heating or an HF-welding when it is a question of welding plastic materials presenting a sufficiently high dielectric loss, such as in case of PVC.
The HF-welding is advantageous in only using a single electrode, i.e. the welding electrode, whereas the heating of the material is caused by the dielectric losses in the material with the result that the method is very easy and convenient to perform and does not require a production of exactly mating and accordingly expensive electrodes. The HF-generator of the machine is, however, very expensive and involves a high consumption of energy. The method has gradually been extensively used especially for welding PVC.
A heat contact welding involving an indirect heating of the plastics employs usually a welding electrode and a counter electrode, where both electrodes are heated, the welding electrode being heated to a temperature about the softening temperature of the plastics and the counter electrode being heated to a temperature exceeding the melting temperature of the plastics. The welding electrode includes the particular welding profile acting directly on the possibly Teflon-coated top side of the plastics, whereas the counter electrode is plane and shielded from the plastics by means of a separation tape of a higher melting point than the plastics. The latter method has been extensively used for welding thin sheets, but can only be used to a limited extent for welding thick sheets and materials because the heat must be conducted through the plastics in order to reach the welding spot, and accordingly the top side of the plastics is heated more than the welding spot, which causes problems of heat damages of the plastics outside the welding seams. In order to obtain high production rates, attempts have been made at operating at temperatures being as high as possible, but such procedures only made the situation worse because the high temperatures and high amounts of heat, especially in the bottom tool, involved a considerably increased risk of the sheets being damaged. A consequence of the unintended heating is that the sheets turn soft and form bulges, especially in connection with even very short breaks. Attempts have been made at solving this problem by establishing suitable screening devices and cooling devices, but these devices did not provide satisfactory results when thick sheets were involved.
The above problems have become particularly topical because of the present requirements for restricting the use of PVC so as to reduce the environmental damages. Accordingly, a demand has arisen for using other types of plastics, especially polyolefines. These types of plastics cannot, however, like PVC be welded by way of HF-welding in a satisfactory manner, and accordingly it is necessary to use heat-contact welding although such a welding does not provide satisfactory results.
The problem applies in particular to the use of polyolefines for the manufacture of thick-walled products, such as loose-leaf binders or more composite products, such as various wrappers comprising a varying number of layers of material and consequently of a varying thickness on various locations of the product. In particular the use of polypropylene causes problems because said material presents a very narrow temperature range (approximately 8.degree. C.) for being weldable.
Pulse heat welding has furthermore been used within the packing industry, especially for the manufacture of bags, where the heating is carried out by means of electrically heated resistance wires. This method is, however, only used for thin sheets, and as far as the appearance is concerned the quality of the weldings has been very poor compared to the requirements that can be met by HF-welding PVC.
In EP-A-335 270 is disclosed a welding press for welding of heat-weldable material by means of indirect heating, and including a welding electrode and a counter electrode, both being electrically heated permanently and comprising thermostats (not shown). A heat-conducting separating tape is separating the counter electrode from the material. The temperature control is insufficient for welding of polyolefines of more than 100 .mu.m.
In DE-A-21 27 539 is disclosed an apparatus comprising pulse heating wires secured to the counter electrode. Variations of the termperature across the width of the pulse heating wires is disclosed, however, no disclosure of the temperature control is available. The apparatus is unable to provide an embossing or a parting seam.
For a long time the opening of various types of packing made of heat-weldable sheet material of a thickness of approximately 50 to 100 .mu.m has furthermore been a growing source of irritation to the users, because it is almost impossible to open such packings by means of the fingers alone. The latter problem applies to a wide range of packed articles including building materials, feedstuff and fertilizers, furniture, tools , household articles, clothes, toys, gift articles, food, stimulants etc. It is often necessary to use tools, such as scissors or knives, with the result that injuries easily arise to the packed article or the user. It has not previously been possible to provide a satisfactory solution of this problem by the known welding methods.